Airborne reconnaissance systems have been widely used for many years now, particularly for obtaining images from the air of areas of interest.
Originally, a film camera was used on board of the aircraft for capturing images of the terrain. The main problem of an airborne, film-camera based reconnaissance system is the length of time required for developing the film, an operation that can be performed only after landing. This problem has been overcome in more modern systems by the use of a one-dimensional vector or a two-dimensional array of light-sensitive sensors (generally such an array is called a “focal plane array” hereinafter also referred to as FPA) in the camera for obtaining electronic images that are then electronically stored within the aircraft, and/or transmitted to a ground base station. This is generally done in such systems by scanning of the area of interest by the light-sensitive sensors in the direction of the flight.
Airborne reconnaissance systems are generally used to obtain images of hostile areas, and therefore the task of obtaining such images involves some particular requirements, such as:    1. Flying the aircraft at high elevations and speeds in order to reduce the risk of being targeted by enemy weapons, and in order to widen the area captured by each image;    2. When carrying out reconnaissance in low height and/or high speed (i.e., a high V/H ratio), trying not to compromise the image quality.    3. Trying to capture as much relevant image information as possible during as short as possible flight;    4. Trying to operate under various visibility conditions, while not compromising the resolution of the images and their quality.    5. Trying to photograph rough terrains (e.g., high mountains, areas having sharp ground variations), in high resolution and image quality.
The need for securing the reconnaissance aircraft, while flying above or close to hostile areas has significantly increased flying costs and risks, as sometimes the reconnaissance mission requires escorting of the aircraft by other, fighter aircrafts. Therefore, the need for enabling as short and reliable mission is of a very high importance.
There are several other problems that are generally involved in carrying out airborne reconnaissance. For example, capturing images from a fast-moving aircraft introduces the need for the so-called Forward Motion Compensation (Hereinafter, the term “Forward Motion Compensation” will be shortly referred to as FMC), to compensate for aircraft movement during the opening of the camera shutter (whether mechanical or electronic; in the latter case, the opening of the camera shutter for the purpose of exposure is equivalent to the integration of light photons by the light-sensitive components).
When light-sensitive sensors are used in the camera, three major scanning types are used:    i. The Along-Track Scanning (also known as “push-broom scanning”)—In a first configuration of the Along-Track Scanning, the light-sensitive sensors are arranged in a one-dimensional vector (row), perpendicular to the flight direction. The scanning of the imaged area is obtained by the progression of the aircraft. In one specific configuration of Along-Track Scanning, generally called Along-Track TDI (Time Delayed Integration) configuration, a plurality of such parallel one-dimensional vectors (pixel-rows) perpendicular to the flight direction are provided at the front of the camera forming a two-dimensional array. In that case, however, the first row of the array captures an area section, while all the subsequent rows are used to capture the same section, but at a delay dominated by the aircraft progression. Then, for each row of pixels, a plurality of corresponding pixels of all the rows in the array, as separately measured, are first added, and then averaged in order to determine the pixel measured light intensity value. More particularly, each pixel in the image is measured N times (N being the number of rows) and then averaged. This Along-Track TDI configuration is found to improve the signal-to-noise ratio, and to improve the image quality and the reliability of measuring.    ii. The Across-Track Scanning (also known as “Whiskbroom Scanning”)—In the Across-Track Scanning, a one-dimensional sensing vector of light-sensitive sensors, arranged parallel to the flight direction, is used. The sensing vector is positioned on gimbals having one degree of freedom, which, during the flight, repeatedly moves the whole vector right and left in a direction perpendicular to the direction of flight, while always keeping the vector in an orientation parallel to the direction of flight. Another Across-Track Scanning configuration uses a moving mirror or prism to sweep the line of sight (hereinafter, LOS) of a fixed vector of sensors across-track, instead of moving the vector itself. In such a case, the Across-Track Scanning of the area by the gimbals having one degree of freedom, while maintaining the forward movement of the aircraft, widens the captured area. Another configuration of the Across-Track Scanning is the Across-Track TDI configuration. In this configuration there exists a plurality of vectors (columns) in a direction parallel to the flight direction, forming a two-dimensional array. This Across-Track TDI, in similarity to the Along-Track Scanning TDI, provides an improved reliability in the measuring of pixel values, more particularly, an improvement in the signal-to-noise ratio.    iii. Digital Framing Scanning: In Digital Framing Scanning, a two-dimensional array of light-sensitive sensors is positioned with respect to the scenery. In U.S. Pat. No. 5,155,597 and U.S. Pat. No. 6,256,057 the array is positioned such that its column-vectors (a column being a group of the array's columns) are parallel to the flight direction. Forward motion compensation (FMC) is provided electronically on-chip (in the detector focal plane array) by the transferring of charge from a pixel to the next adjacent pixel in the direction of flight during the sensor's exposure time (also called “integration time”). The charge transfer rate is determined separately for each column (or for the whole array as in U.S. Pat. No. 6,256,057 where a slit is moved in parallel to the columns direction), depending on its individual distance (range) from the captured scenery, assuming flat ground. In WO 97/42659 this concept is extended to handle transferring of charge separately for each cell instead of column, a cell being a rectangular group of pixels. In the system of U.S. Pat. No. 5,692,062, digital image correlation between successive frames captured by each column is performed, in order to measure the velocity of the scenery with respect to the array, and the correlation result is used for estimating the average range of each column to the scenery, for the purpose of motion compensation in terrain with large variations. This compensation method requires capturing of three successive frames for each single image, two for the correlation process and one for the final motion-compensated frame. The system of U.S. Pat. No. 5,668,593 uses a 3-axis sightline stepping mechanism for expanding coverage of the area of interest, and it applies a motion compensation technique by means of transferring of charge along columns. U.S. Pat. No. 6,130,705 uses a zoom lens that automatically varies the camera field of view based on passive range measurements obtained from digital image correlation as described above. The field of view is tuned in accordance with prior mission requirements for coverage and resolution.
A significant problem which is characteristic to all the above types of prior art reconnaissance systems is the limited field of view of the camera. Generally, the prior art systems comprise a lens at the front of the imaging system, and additional optics which impinge the terrain image onto a focal plane array. The lens generally has a limited field of view, in the range about 30°. Increase of the lens field of view results in an image of a larger area portion, but also in reduction in the resolution of the captured image. Therefore, when there is a need to obtain high resolution images in a wide field of regard by the prior art systems having a limited field of view, in most of such systems the optics and the focal plane array are mounted on gimbals, which repeatedly change the camera line of sight direction towards the terrain. The line of sight is changed in a direction perpendicular to the flight, i.e., from the right to the left of the aircraft and vice versa. During each such change of the line of sight transversal to the flight direction, hereinafter also referred to as a “a scanning step”, one terrain image from a transversal terrain strip is acquired. Performing the scanning steps in a fast and accurate manner requires having a complicated gimbals system and servo mechanism. The more such steps are included in a single full strip scanning (a full strip scanning is defined as the scanning by which a full transversal terrain strip is obtained), results in more images, each image referring to a smaller terrain area, but with higher resolution.
Generally, the terrain area which is acquired by each image capturing is also a function of the focal plane array resolution. When the focal plane array includes more pixels, a larger area portion can be acquired for a given resolution. When the number of the pixels in the focal plane array is small, image of a smaller terrain area is acquired, and more scanning steps are required in order to cover a given area at a given resolution. However, more scanning steps involve more stress on the gimbals system, or alternatively, enforces slower scanning. There are many cases in which the optics in front of the FPA can enable obtaining a larger area with a satisfactory manner, but the small focal plane array (i.e., having fewer pixels) enforces a smaller terrain area image and more scanning steps.
It is therefore an object of the present invention to reduce the number of scanning steps required for scanning a given terrain area in a desired resolution.
It is another object of the present invention to enable scanning a larger area in a given time and resolution, but with a smaller focal plane array.
It is still another object of the invention to reduce stress and accuracy requirements from the gimbals system on which the optics and focal plane array are mounted.
Other objects and advantages of the present invention will become apparent as the description proceeds.